Currently, storage systems using multiple storage devices such as HDDs (Hard Disk Drives) are widely used. In such storage systems, the safety of stored data is enhanced by controlling data recording in such a manner that the data are redundantly stored in more than one storage device by using the RAID (Redundant Arrays of Inexpensive Disks) technology.
In addition, in the storage systems in which data are redundantly stored, when a storage device fails, data stored in the failed storage device are rebuilt in another storage device. Such processing is called rebuilding. When rebuilding is performed, the redundancy is restored.
Incidentally, many storage systems are provided with a standby storage device called hot spare, and the rebuilding is performed by using the hot spare in many cases. On the other hand, according to a rebuilding technique proposed for the storage systems not containing the hot spare, when a storage device belonging to a first RAID group fails, one of storage devices belonging to a second RAID group (which is configured to redundantly store data) is separated from the second RAID group, and rebuilding is performed by using the separated storage device. (See, for example, Japanese Laid-open Patent Publications Nos. 2008-197886 and 10-260789.)
In the case where one of the storage devices belonging to the second RAID group is separated from the second RAID group and rebuilding is performed by using the separated storage device as above, the use of the second RAID group is resumed after the RAID level is changed and the data redundancy is lowered. Further, in the case where the data in the second RAID group are managed in such a manner that the parity data are distributedly recorded in accordance with, for example, RAID 5 or RAID 6, the data are rebuilt and rearranged when the RAID level is changed. However, the processing for rebuilding and rearrangement of the data may affect the processing for access to the storage devices in response to I/O (In/Out) requests from a host apparatus, and lower the speed of the processing for access.